1. Field of the Invention
The present invention relates to a hydrometallurgical process for the extraction and recovery of silver, copper, zinc, lead, antimony, iron, and sulfur from complex high-grade silver-bearing sulfide ores and flotation concentrates, plus the recovery of the chemical reagents used in the process and the production of zinc ammonium sulfate (ZAS). More specifically, the invention involves an acidic oxidation pressure leach of finely ground silver-bearing sulfide ores and concentrates, recovery of silver, iron, copper, zinc, and sulfur from the pregnant leach solution, and the recovery of lead, sulfur, antimony, and any residual silver from the tails. Chemical reagent inputs are recovered as fertilizer products. The instant invention is non-polluting, with minimal solid waste and no liquid waste requiring disposal, and offers high recovery of silver and other metallic values.
2. Description of Prior Art
Silver-bearing ores and concentrates from many mining districts around the world often contain toxic metals such as arsenic, antimony, and bismuth in addition to valuable lead, zinc, and copper. The silver contained in these concentrates is often in the form of sulfosalt minerals of arsenic and antimony, such as tetrahedrite, tennanite, stephanite, pyrargerite, and other complex minerals normally considered refractory to traditional hydrometallurgical processes, such as lixiviation with cyanide.
The few lead smelters in the world that currently purchase and process ores containing the above-mentioned toxic metals penalize such ores and concentrates to the financial detriment of the producer. The pyrometallurgical methods employed in a present-day lead smelter are little changed from a century ago. In a lead smelter, silver-bearing ores and concentrates are mixed with galena concentrates and oxidized by roasting or sintering in order to drive off the sulfur and volatile metals. The resulting sulfur dioxide gas and oxides of arsenic and antimony are pollutants that must be removed from the gas stream, often at significant cost. The calcine or sinter is then smelted in a blast furnace along with carbonaceous material to reduce the metal oxides to a metallic alloy. The gas stream from the blast furnace must be further treated to remove air-borne pollutants. The metallic alloy obtained from the blast furnace must then undergo additional pyrometallurgical treatment to separate silver from lead.
Research has been conducted during the last several decades to develop an oxidation pressure leach method to process complex silver-bearing sulfide ores and flotation concentrates, since such a method could be scaled to any size operation, the capital costs would be a fraction of the cost of a traditional pyrometallurgical smelter, and the plant could be situated near the mine or mill and operated by even a modest-sized company. Until the present invention, no proposed method has been able to process every type of complex silver ore, recovering all metallic values, using only small amounts of acid and other chemical reagent inputs, and without producing objectionable emissions, nor solid and liquid wastes needing disposal. A leach circuit employing a modification of U.S. Pat. No. 5,096,486 was installed in Idaho to process silver-bearing complex sulfide ores (previously processed with alkali to remove antimony) using a nitric acid/sulfuric acid/oxygen pressure leach process, but that circuit was discontinued because, among other reasons, operational costs exceeded the simple alternative of selling concentrates to a near-by smelter. At the present time, there are no plants employing nitric acid pressure leach methods to process silver-bearing complex sulfide ores.
Raudsepp, et al, (U.S. Pat. No. 4,647,307) teach a method to solubilize iron, arsenic, sulfur, and some silver from gold and silver-bearing pyrites and arsenopyrites using an oxidized nitrogen species such as nitric acid under conditions of heat and oxygen overpressure. Raudsepp, et al, proposed that any silver in solution be precipitated as silver thiocyanate and that the pregnant leach solution then be cooled to remove iron and arsenic, which had to be disposed of in some manner. It was assumed that gold and the remaining silver in the tails would be recovered by conventional cyanide lixiviation. Simple gold-bearing sulfide minerals such as pyrite and arsenopyrite rarely contain more than small amounts of gold and silver per ton of concentrate, thus the method proposed by Raudsepp, et al, not only was un-economic for use with such low-grade ores and concentrates, it did not address the metallurgical problems associated with extracting silver from high-grade, complex silver-bearing sulfide ores, such as the bulk of silver concentrates traded on the world market today.
Posel (U.S. Pat. No. 4,038,361) teaches a method to extract copper and silver from relatively simple sulfide ores, wherein large quantities of nitric acid (from 600 to 800 kgs of nitric acid per 1,000 kgs of concentrate) were used to oxidize the ore. The solids to liquids ratio was 1:10. Posel proposed that the NOx gases resulting from the reaction of nitric acid with ore be passed through an oxidizer and absorption column in order to regenerate nitric acid. Temperatures had to be carefully maintained so that sulfur would form beads during cooling, which beads could be separated from the leach slurry by screening. However, only the most simple sulfide ores respond to the Posel method with anything approaching the high silver recoveries described. In reality, silver contained in complex sulfide ores does not dissolve at the levels (99%+) reported by Posel even using high amounts of nitric acid, but rather dissolve at lower levels due to the formation of insoluble silver compounds. The beaded sulfur that Posel described entrains significant quantities of pregnant solution and will assay as much as 3% of the silver contained in the solution. This entrained silver is difficult to separate from the beaded sulfur. Moreover, any error in the cooling of the leach slurry, or a variation in the mineralogy of the ore, will result in the formation of “gummy” sulfur, which has a tendency to precipitate almost all of the silver from the pregnant solution due to an electrochemical exchange between elemental sulfur and unreacted pyrite in the slurry, causing the electro-deposition of silver on the surface of the elemental “gummy” sulfur. The silver contained in the “gummy” sulfur would require considerable additional processing in order to be separated. Furthermore, even were error completely overcome and the formation of “gummy” sulfur eliminated, not all the elemental sulfur will form beads of sufficiently large size to be collected by a screen; a significant portion of the elemental sulfur will report to the tails as finely divided sulfur, which would complicate the disposal of, or further processing of, the tails. No consideration is given in the Posel method for the elimination of arsenic from leach solutions—which arsenic would render the ammonium nitrate produced by this method unsuitable for sale. All the lead and antimony contained in the mineral concentrates would report to the tails as potentially toxic compounds, making untenable the statement by Posel that such tails could be sold to the road-building industry.
Kunda (U.S. Pat. No. 4,331,469) teaches a dual leach process, wherein complex silver ores would be treated at relatively low temperature and pressure with very high concentrations of nitric acid in an autoclave and then retreated with even higher levels of nitric acid to dissolve insoluble silver compounds. The solids to liquids ratio was about 1:7. Claims were made of 98% recoveries of silver. However, diligent investigation shows that Kunda's assertions of high silver recovery using the method described cannot be duplicated in silver-bearing concentrates where the preponderant mineral is any one of the sulfosalts containing antimony, which would include many of the complex silver ores produced around the world. Under the conditions described by Kunda a percentage of silver (25 percent or more) would be converted to insoluble compounds in the initial oxidation leach-which compounds are completely refractory to further acidic leaching. The method described by Kunda could only be used on a specific type of relatively non-complex silver concentrate and is not useful for the treatment of complex silver ores, especially those containing antimony. Kunda describes the precipitation of zinc sulfide from a pH neutral solution, however, it is well known to those with a passing knowledge of the art, that under such conditions, zinc sulfide is extremely slimy and requires extraordinary effort to filter. Another drawback to the method described by Kunda would be the presence of finely divided elemental sulfur in the oxidized tails of the leach reaction, plus insoluble oxide compounds of lead and antimony. This elemental sulfur would be difficult to remove and would hinder any attempt to further recover lead or antimony from such tails by either pyrometallurgical or hydrometallurgical methods. None of the oxidized metallic compounds contained in the tails could be released to the environment, but would require further treatment, which Kunda does not contemplate.
Anderson, et al, (U.S. Pat. No. 5,096,486.) teach a method wherein silver-bearing concentrates were treated under conditions of mild temperature and pressure, using oxygen, sulfuric acid, water, and sodium nitrite in an autoclave, thereby dissolving about 92% of the silver. The solids to liquids ratio was described as approximately 1:9. The pregnant leach liquor containing iron, arsenic, zinc, copper, etc., would be treated with solvents to extract a majority of the copper. The remaining dissolved metals, including iron, copper, zinc, and arsenic, and the high amounts of sulfate in the leach solution would be treated with lime and discharged to a tailings pond. The environmental stability of the solids produced by such lime treatment is the subject of debate. Because of the risk of arsenic and other toxic metals leaching from tailings ponds into the environment, and the undeniable fact that tailings ponds have been known to break with dramatically negative consequences for the environment, operations that require tailings ponds may be restricted in the future. Among the problems associated with the method proposed by Anderson, et al, are i) the very large quantity of sulfuric acid used in the pressure leach (1,931 kgs of sulfuric acid per 1,000 kgs of concentrate) that must be later neutralized at relatively high cost and disposed of into the environment; ii) the objectionable discharge of possibly unstable toxic compounds into the liquid phase of a tailings pond, from which they might leach into the environment; iii) the presence of elemental sulfur in the tails; and iv) the presence of lead sulfate and oxides of arsenic and antimony in the tails.
McElroy, et al, (U.S. Pat. No. 3,856,913) reveal a method for leaching simple copper-bearing sulfide concentrates in which silver salts are added to the slurry to improve the amount of copper dissolved in the leach medium. McElroy, et al, propose that between 0.2 kg and 1 kg of silver salt be added for every 1000 kgs of concentrate. The added silver would report to the tails as insoluble silver compounds and McElroy, et al, propose that the tails subsequently be leached with cyanide to recover silver. However, the use of silver salts in the manner proposed by McElroy, et al, is both uneconomical and impractical, given that the silver would report to the tails as insoluble compounds. The cost of leaching silver from the tails would offset any gains in the initial rate of copper dissolution. Given that, per weight basis, silver is a precious metal at least 100 times more costly than copper, to send such amounts of valuable silver into tailings would be commercially unthinkable. For these and other obvious reasons, no embodiment of the process proposed by McElroy has ever been put into operation.